This question already has an answer here:
The technique of "mathematical induction" is a method of proof where you show some theorem is true for some starting integer and prove also that it holding at any arbitrary integer implies it must hold at the following integer. From this you conclude it must hold for all integers from the starting point on.
I think most people would consider this "obvious". So obvious in fact that I always considered it as dating back to the earliest history of mathematics, such as to the ancient Greeks. But I was teaching a course in Discrete Math and was surprised to read in the text that the first explicit explanation of the principle was from a mathematician in the 1500's named Francesco Maurolico: http://en.wikipedia.org/wiki/Francesco_Maurolico
How could it have taken so long for mathematicians to discover it? It makes you wonder if there are also other obvious principles out there that have not been yet explicitly enunciated.
Another point: European math and science in general was in deep decline during the Dark Ages. But Chinese and Indian and Arabic math continued to make great advances during this period. I wouldn't be surprised if we had greater records of the mathematicians in these other cultures during that period that they had stated the principle earlier.